Various systems have been designed to detect and classify occupants of a predetermined volume. For example, detection and classification systems have been utilized in the field of security to detect intruders and in the field of safety to determine whether, for example, a human being is trapped within a predetermined volume. Detection and classification systems have also been utilized in personal health and medical applications. A variety of systems have been proposed and/or developed for the purposes of monitoring a predetermined volume, such as a room of a building, a vehicle interior and/or other defined volumes. One such system has implemented ultrasonic technology to determine the presence of a human being within a predetermined volume. In general, such ultrasonic systems implement acoustic wave signals with frequencies above the normal range of human hearing. Many such systems have transmitted ultrasonic signals in a pulse mode and analyzed a returning reflected signal using a time-of-flight concept. Other ultrasonic systems have employed continuous transmission and have analyzed returning reflected signals for a Doppler shift.
While many known systems are successful in certain applications, within certain limitations, in detecting a moving occupant and generating an alert that is interpreted as detection of a living occupant, such systems have generally been hampered by their inability to deal with problems in acoustical impedance of various transmission media. Generally, such systems have also had difficulty in dealing with wave interference caused in received signals, which are composite signals that include portions of both a transmitted and reflected acoustic wave. As such, prior art systems have generally been limited to the detection of gross body movements and, as a result, have not been sufficiently sensitive to reliably detect small movements characteristic of a respiration of a human being, which involves only relatively small body movements.
What is needed is a technique that is capable of accurately monitoring a respiration rate of an occupant of a predetermined volume. It would also be desirable if the technique had the ability to acquire, lock and continually track the respiration rate of the occupant, especially when other body movements are present.